Introduction
Material selection is one of the most consequential decisions in CNC machining—and one of the most underestimated. Select the wrong material, and you’re inviting tool wear, lost tolerances, extended lead times, and skyrocketing production costs. Worst of all, you’re putting at risk your part’s chances of surviving in the field.
The right material will define cycle time, surface finish, post-process requirements, and your part’s ability to function as engineered when exposed to real-world operating conditions.
This guide will help you make smart material decisions every time — whether you’re prototyping a medical device, machining an aerospace component, or running high-volume automotive production.
Why Does Material Selection Matter?
Every material behaves differently under cutting forces. No materials react the same way to cutting forces. Material hardness directly impacts tool life. Ductility can impact chip formation. Thermal conductivity can cause heat buildup on the cutting edge. These concerns have little to do with theory; they have everything to do with your bottom line.
Material selection impacts:
- Machining time: Cutting speeds are reduced on harder materials, which also may require multiple passes to achieve desired tolerances
- Tool wear: Tough, abrasive, or work-hardening materials like titanium, Inconel, or stainless steel will kill your cutting tools
- Surface finish: Many materials naturally produce a decent finish after machining, while others require careful control to prevent melting or burring.
- Part performance: Just because a material machines well doesn’t mean it will hold up under service conditions. Make sure your material can handle the loads, corrosion, and temperatures it will face
- Total cost: While material price is important, machinability is the hidden multiplier of material cost
Buying a cheap material that takes three times as long to machine will never be the most cost-effective solution.
Factors Affecting CNC Material Selection
Strength
First and foremost, determine what the CNC part needs to do. What is the minimum tensile strength, yield strength, hardness, and fatigue strength you need based on the real loads the part will experience? Specifications greater than you need and you are throwing money away; too few and you will fail.
Machinability
Rated relative to 160 Brinell free-cutting brass as 100%, machinability ratings give you an idea of how easily and quickly a material can be cut. Ease of machining determines how fast you can go, how long your tools will last, and the surface finish you achieve. All affect your cost per part.
Price and lead time
While the raw material price is important, so is lead time. That dream alloy you found might work perfectly on paper, but what are your options for manufacturing if it’s only available as a block? You can get aluminum or steel bars, rods, tubes, and more in any size from just about anywhere in the world.
Density
It is something that comes up a lot with aerospace and automotive clients. Don’t they care about strength? Of course, they do, but they care about how much a material weighs just as much as they care about how strong it is. Aluminum is a good choice when you need strength with a low weight. Titanium pushes the boundary even further. Use steel and brass only if their strength allows you to get away with their weight.
Environment
What will your part be exposed to in the field? Salt water? Use stainless steel, anodized aluminum, or maybe even plastic. Indoor gearboxes and mechanical components? You don’t need anything more than carbon steel with a surface coating. Nobody wants to spend the time and money replacing parts that should have lasted when interacting with the environment they were designed for.
Temperature
Will your part be subjected to high temperatures during operation? Aluminum loses strength above 150°C for structural applications. Certain plastics, like PEEK, can withstand temperatures up to 250°C. Steel and titanium can withstand pretty extreme temperatures.
Application Requirements
There are going to be times where the application your part will be used in will limit your material choice. Medical devices need to be made from biocompatible materials. Parts for aerospace applications usually need to be made from material that has been certified by the customer (CCI, AS9100), which requires material traceability.
Overview of Common CNC Materials and Applications
Aluminum
Density: 2.7 g / cm³; very good machinability, good corrosion resistance, and high thermal conductivity.
Advantages: Very fast feeds and speeds, long tool life, light weight and anodizes well. 6061-T6 and 7075-T6 grades can be used for most applications requiring structural strength.
Disadvantages: Not as strong as steel; softer surface; and galling can occur under high contact pressures.
Typical applications: aerospace machine housings, automotive brackets, consumer electronic cases, robotics parts, and prototypes.
Steel (Carbon, Alloy)
Properties: Very high strength, excellent machinability (when using proper grades), ubiquity, and heat treatability.
Advantages: Inexpensive when used for structural applications, available in many grades offering a wide range of strength options, and excellent machinability when using properly selected grades such as 1018 and 4140.
Disadvantages: Heavier than aluminum, rusts if left untreated and harder grades are more expensive to machine.
Typical applications: Shafts, gears, structural parts, tooling/molds/jigs and fixtures.
Stainless Steel
Properties: A wide variety of chromium-nickel alloys. Moderate to high strength, tends to work harden.
Advantages: Stainless (rust-proof), hygienic surface ideal for medical or food-contacting applications.
Disadvantages: Work hardens quickly during machining. Sharp tooling and carefully selected cutting parameters are required. Generally more expensive than carbon steel.
Typical applications: medical devices, food processing components, marine hardware, and pharmaceutical equipment.
Brass
Properties: Copper-zinc alloy, excellent machinability, excellent electrical conductivity, naturally corrosion-resistant.
Advantages: Machines faster than nearly any other metal, excellent surface finish right out of the machine, and does not produce a built-up edge on tooling.
Disadvantages: Relatively expensive, heavy, not recommended for structural applications requiring high strength.
Typical applications: fittings, valves, electrical connectors, precision fasteners, and decorative housing hardware.
Engineering Plastics (ABS, Nylon, PEEK)
Properties: Each plastic grade has unique properties. Mechanical and thermal properties vary between grades, light weight, chemically resistant electrical insulator.
Advantages: Machines are easy/fast, ABS and nylon are very inexpensive, and certain grades, such as PEEK, are biocompatible and chemical resistant.
Disadvantages: Not as strong as metals; poor dimensional stability when exposed to moisture over time (e.g., nylon); and achieving a good surface finish can be difficult on some grades.
Typical applications: Electrical/mechanical enclosures, insulators, bushings, medical implants (PEEK), consumer goods, and jigs.
Titanium
Properties: High strength, outstanding corrosion resistance, biocompatibility, and low thermal conductivity.
Advantages: Strongest metal that won’t weigh you down.
Disadvantages: Low thermal conductivity leads to heat buildup at the tool tip. Tool wear is high, and machining speeds are low. It’s a very expensive material and costs a lot to machine.
Typical applications: Aerospace structural components, surgical implants, high performance automotive applications, and marine.
Material Comparison
| Material | Machinability | Strength | Weight | Corrosion Resistance | Relative Cost |
| Aluminum 6061 | Excellent | Medium | Low | Good | Low |
| Carbon Steel 1018 | Good | High | High | Poor | Low |
| Stainless Steel 316 | Moderate | High | High | Excellent | Medium |
| Brass C360 | Excellent | Medium | High | Good | Medium |
| PEEK | Good | Medium | Low | Excellent | High |
| Titanium Grade 5 | Poor | Very High | Low-Med | Excellent | Very High |
Material Selection Mistakes to Avoid
Choosing based only on material cost.
The cheapest material per kilogram becomes pricey when it doubles your cycle time or wears through tooling every 20 parts.
Ignoring tolerance requirements.
After being machined, moisture-absorbing plastics undergo dimensional changes. It is standard practice to set strict tolerances for hygroscopic materials without considering environmental stability.
Overengineering the material choice.
Using titanium where 6061 aluminum will perform properly adds cost, complexity, and lead time with no functional benefit. Instead of choosing the best option, match the material to the requirement.
Neglecting post-processing compatibility.
Anodizing works on aluminum, not steel. Hard chrome plating is restricted in many jurisdictions. Confirm your intended surface treatment is compatible before locking in a material.
Post-processing Considerations
Anodizing is a surface treatment that works for aluminum, but you can’t anodize steel. Hard chrome plating is becoming restricted in many countries. Verify that your intended surface finish can be applied to your material.
Expert CNC Material Selection Tips
Answer these questions first, then pick a material that meets these requirements.
- Request material certifications when material traceability is important. For aerospace and medical applications, a material test report (MTR) isn’t a requirement — it’s standard best practice.
- Material selection should occur during the design stage. If you have control over the part geometry, then design features that are friendly to your material.
- Don’t rely on material selection alone to provide corrosion protection. Consider coatings, surface treatments, and even compatibility of materials used in assembly when specifying your parts.
- Consult your CNC machinist when selecting material. Talking to your machine shop before you finalize your material selection can save you a lot of frustration.
Conclusion
Selection of materials is not a technical aspect but rather the basis of your part’s performance, cost, and manufacturability. The distinction between an effective project and an expensive mistake often comes down to the choice of material that meets the requirements of the environment and practical application.
Don’t know what kind of material is appropriate for your project? Receive the professional advice of the FST Metals team and select the best high-performance solution at the most affordable cost from the start of the project.
FAQs
What’s the easiest material to CNC machine?
Due to their low cutting forces and ability to produce consistently cool chips, brass (C360) and aluminum (6061) are universally the easiest materials to machine.
What’s the cheapest material to CNC machine?
When combining raw material price with ease of machining, carbon steels (1018, 1045) and aluminum 6061 are among the cheapest materials to machine. ABS plastic is another material that’s cheap to buy, but its machining characteristics will vary.
What material has the highest strength?
Titanium Grade 5 (Ti-6Al-4V) has the best strength-to-weight ratio of any metal. If weight is not a factor, then there are many tool steels and alloy steels that offer higher strength. 4340 steel is a very strong material used in many demanding applications.
How does material selection impact my CNC machining cost?
Material affects costs in three ways. The raw material itself has a cost per kilogram, but some materials just take longer to machine than others. Some materials wear your tooling out faster, too. A material that costs twice as much per kilogram but machines 3x faster could end up being the cheaper material at scale.
